Method and device for the production of blown hollow bodies

ABSTRACT

The present invention refers to an arrangement for the production of blow-molded hollow bodies from plastic with a blow molding machine, wherein a cooling medium can be blown via a conduit into a blow mold respectively into the hollow body for blowing up and cooling of the blow-molded hollow body. For improvement of cooling the interior of the hollow body within the blow mold, a closed conduit system is provided, in which the cooling medium circulates and is kept at an elevated pressure level and is cooled by means of a heat exchanger. Thereby, a higher yield of piece per time is realized at lower production cost. The closed cooling medium circulation system can be essentially retrofitted to all blow molding machine in order to raise efficiency.

[0001] The present invention refers to a process and an arrangement forproducing blow-molded hollow bodies from plastic by means of ablow-molding machine, wherein preferably a gaseous cooling medium can beblown into the blow mold or into the hollow body for blowing the hollowbody and cooling the blow-molded body via a conduit. After blowing thehose shaped pre-form made of moldable plastic material into the actualhollow body, the final blow-molded hollow body or the plastic productmust subsequently remain in the mold and cooled until it has reached asufficient inherent stability and can then be taken from the mold. Thetime in which the hollow body is cooled is defined as the coolingperiod.

[0002] The cooling procedure of the plastic hollow body in the blow moldis generally the slowest and is thus the time-determinative step in theproductivity of a blow molding machine. When reducing the coolingperiod, the efficiency or the number of discharged pieces of the articleproduced per time unit can be raised considerably. Cooling of the hollowbody within the blow mold is carried out on the one hand from theoutside through a direct contact of the plastic material with theinterior wall of the cooled blow form and on the other hand, from theinside via the cooling medium which is blown in. According to the priorart, cooling is understood to mean all methods where heat is withdrawnfrom the inner wall of the blow molding piece. Thereby, a heat transfertakes place from the inner side of the surface of the hollow body to thecooling media that are within the hollow body and a subsequent heattransport away therefrom. The energy which is taken up through the mediais taken from the hollow body and together with the media is dissipatedto the outer environment.

PRIOR ART

[0003] The methods known in the prior art for inside cooling of theblow-molded plastic hollow bodies vary in the way and the condition ofthe medium or media used and a phase transition which eventually takesplace and the type of exchange between the interior of the hollow bodyand the environment. Normally air that has been compressed (compressedair) is utilized as a medium. In the so-called dynamic air process, thehollow body is blown up by means of compressed air which is blown intothe pre-form. The blow pressure is normally between 5 to 8 bar. Thecompressed air trapped within the hollow body remains there at first,takes up heat from the hollow body interior and during venting, is beingtransferred to the environment. Thereby a certain amount of heat iswithdrawn from the hollow body. The so-called interval blow moldingprocess works in a similar way. In this process, the enclosed and heatedair is allowed to escape in intervals, which then is replaced throughcompressed air with lower temperature. In the back flush or blow airprocess, the exchange of warm air through cold air is carried out in acontinuous process in the form of a back flush procedure. It is possibleto intensify the known method by cooling the blow air utilizedpreviously, for example normal compressed air temperature of 25 C to 0 Cor, after preceded drying of the blow air (for example by means of anadsorption dryer) or to cool further to below 0 C, for example, to −30C. Thereby, a greater temperature difference is realized between thecompressed air and the interior surface of the hollow body. In addition,the blow pressure can be raised from a level which is significantlyabove the normal level and can be considered as a separate variation ofthe interior cooling. Further cooling methods that are known inaccordance with the prior art, are interior cooling with deeply cooledliquefied gases. Thus, for example after blowing up the pre-from withthe compressed air, deeply cooled liquefied carbon dioxide or nitrogenis blown into the hollow body as an additional medium. The liquidcomponents that evaporate warm up under uptake of considerable amountsof heat. A further possibility to realize interior cooling is byinjecting water or other media after blowing up the pre-form. Additionof these media can be realized in a continuous way or alsointermittently. Some of the afore-described methods can be also carriedout as combined methods.

OBJECT

[0004] It is therefore object of the present invention to improve theinterior cooling of blow-molded plastic hollow bodies, while at the sametime, lowering overall the manufacturing cost of the manufacturedplastic articles.

[0005] This object is solved by the arrangement having features asrecited in claim 1. The following dependent claims recite furtheradvantageous embodiments of the blow mold arrangement according to thepresent invention. The object of the invention is also solved inaccordance with the features of the process as recited in claim 16. Thedependent claims that are following recite additional advantageousembodiments thereof.

[0006] The arrangement according to the present invention serves asrealization of an interior cooling of the most novel type. The coolingeffect realized in the blow mold or in the hollow body which is blown uptherein follows according to the combination of about three knownmethods, namely that of the blow air method under utilization of cooledcompressed air or compressed air which has been cooled further or deeplycooled compressed air under elevation of the blowing pressure. The blowair or the blow medium can thereby be enriched with a further gaseousmedium (e.g. nitrogen).

[0007] The function of the arrangement according to the presentinvention is summarized in the following way.

[0008] 1. The cooling medium is lead into a closed conduit system whereit circulates and is filtered by means of one or more filters and cooledby means of integrated heat exchangers or cooled even lower by means ofan additional cooling aggregate.

[0009] 2. The circulating cooling medium in the closed conduit system,depending upon the size of the hollow body, is at elevated pressurelevels up to about 20 bar.

[0010] 3. The cooling medium is blown into the hollow body to realize animproved cooling effect by means of a swirl body under a fluidizingrotational flow to thus improve the heat exchange and in order toimprove the cooling effect.

[0011] 4. The heat capacity of the cooling medium is raised throughincrease of the blow air or the partial or complete exchange of othergaseous or fluid media (e.g. carbon dioxide, nitrogen, propane, water,noble gases or other gases which exhibit a heat capacity that is higherthan that of air).

DESCRIPTION OF THE ARRANGEMENT

[0012] The present invention is more closely described and illustratedin FIG. 1 as follows by means of a schematic illustration of anembodiment. The arrangement comprises the blow mold machine 10 with blowmold 12, in which the plastic hollow body is blown up. The blow mold 12is integrated into a closed conduit system 14 for the blow—and coolingmedium or the blow air.

[0013] Normally, the blow and cooling medium is regular air, which asfollows is designated blow-or cooling air. Of course, the blow air canbe enriched or replaced by other gaseous or fluid media (e.g. carbondioxide nitrogen, propane, noble gases or other gases with elevated heatcapacity) It is also possible to gasify or vaporize fluids such e.g.water and or solid media particles (e.g. carbon dioxide ice) foradditional injection into the hollow body. The injection of the blow airinto the hollow body is carried out at the inlet via a blow mandrel 16and at the outlet, likewise via a corresponding blow mandrelrespectively an expanding mandrel 18. When dealing with particularhollow bodies, such as for example a canister, which is equipped withonly one inlet and/or outlet opening, the single blow mandrel isnormally provided with an inlet line and a corresponding outlet line forthe compressed air. The inlet line and the outlet line can be arrangedadjacent to each other in concentric manner in one blow mandrel.

[0014] The conduit system 14 comprises at the inlet side furthercomponents such as a connector 20 to a cooling medium reservoir, a checkvalve 22 a storage container 24, a compressor 26, a filter 28, a heatexchanger 30 or a cooling aggregate, a further check valve 32, a secondstorage container 34 and as proximate as possible directly in front ofthe blow mandrel 16, a cut-off valve 36. At the outlet side likewise asclose as possible directly in front of the blow mandrel 18, a furthercut-off valve 38 with ventilation outlet. A pressure control valve 40, asecond filter 42, a second cooler 44 (=heat exchange) and a furthercheck valve 46 follow thereafter. The conduit system 14 closes the cycleby following downstream of the check valve 46 and the first check valve22. When utilizing a cooling medium whose temperature is below thefreezing point for water, then all components utilized in connectiontherewith can be affected thereby and thus must have a suitable lay-outso that freezing of one of the various components is prevented.

DESCRIPTION OF THE PROCESS

[0015] The cooling medium (e.g. blow air) is taken from the storage 24at a temperature T₁ under pressure P₁. The storage container 24 is keptunder a minimum pressure of P₀ (e.g. 5 bar) via a check valve. Themedium taken from the storage 24 is compressed by means of a compressor26 to a higher pressure (for example 20 bar) and after filtering infilter unit 28 and cooling to a lower temperature in cooler 30, isguided into a further storage container 34. From the storage container34, the pre-form which is situated within the blow mold 12 of blowmolding machine 10 is blown up through the medium into the completehollow body and form-finished. Subsequently, the hollow body, after apossible stop phase is continually flushed by the cooling medium fromthe storage container 34. The blowing up and flushing or cooling iscarried out via the first blow mandrel 16 which is brought into theinlet opening of the hollow body. Through this blow mandrel 16, thecooling medium flows via a nozzle 48 into the hollow body. At the nozzle48, a flow efficient flow screen or a swirl guide plate 50 is provided.This swirl guide plate 50 confers some speed in circumferentialdirection (rotational flow) to the flow of the cooling medium which upto this point was flowing in axial direction. In this manner, the mediumflows into the hollow body for cooling purposes. Subsequently, thecooling medium flows through the outlet line or the outlet blow mandrel18 and the pressure control valve 40 from the hollow body. After themedium has exited the hollow body it is however not released into theenvironment as is generally done, but after filtering in filtering unit42 and cooling in the cooler 44 to a temperature T₁ (for example 15 C)is guided through a further check valve 46 and re-entered to the storagecontainer 24 and thus circulated for repeated utilization. The checkvalves 22, 32, 46 ensure the predetermined flow direction. The pressurecontrol valve 40 together with the compressor control ensures that thepressure within the container does not fall below a predetermined level.For removal of the sufficiently cooled off hollow body from the open-endblow form, the cut-off valves which are located closely behind the blowmandrels 16 and 18 are briefly blocked and the blow form vented(depressurized). The loss in cooling medium which occurs thereby isbeing offset by the check valve 22 and the connector 20 from the one ofthe cooling medium reservoir.

[0016] In a further embodiment, the blow air which is brought intocirculation is additionally dried in a dryer 52 and further cooled in atleast one cooling aggregate 54 to a temperature from 0 C to −50 C,preferably to −30 C.

[0017] For this purpose, a parallel by-pass line (by-pass) is integratedinto the arrangement in front of the check valve 32 by means of aswitching valve 56. In this by-pass line, after the switching valve 58,the blow medium is dried in a dryer 52 (for example an adsorption dryer)up to suitably low dew point and further cooled in at least one furthercooling aggregate 54. Via a further switching valve 60, the so dried andfurther cooled blow air is coupled into the closed circulation circuit.

[0018] In this embodiment, the hollow body to be produced is firstpre-blown via the normal circulation with blow medium that has not beenfurther cooled, (for example auxiliary air) and then blown; for thecooling phase, the valves 56 and 60 are being changed over and the blowmedium which has been cooled to a lower temperature is guided via theparallel cooling distance into the closed circulation cycle and into theblown hollow body.

[0019] The special feature of the method according to the invention ischaracterized by the following new features and advantages:

[0020] The media which are utilized for cooling are exclusively cycledwith only little loss of the respective pressure level; wherein thepressure level can be up to 20 bar depending upon the size of the hollowbody. The cooling media also serve as heat exchanger and transportbetween the interior surface of the hollow body and a heat exchanger(heat release from the system). Thus, under operation, the compressorhas to compensate for only the relatively small system losses occurringduring pressure release upon discharge of the hollow body from theopened blow form as well as flow losses and expansion work within themedia cycle. As a result, the system operates with a very large flowvolume at high interior pressure level, without substantial use inoperating material of cooling media.

[0021] All techniques known in the prior art still suffer from the samedrawbacks namely that after the transition of heat energy from theinterior of the container to the blow—respectively the cooling medium,the medium itself, especially when run at relatively high pressurelevel, is being released into the environment and thus is not availablefor the further operation and the further use in the cooling process.

[0022] Through the use of a guide screen or a flow guide plate at or inthe nozzle, a rotational momentum is being introduced into the flow.This flow leads to an additional rotational flow and media turbulence inthe hollow body. This likewise leads to an improvement of the heattransfer and heat discharge of the media in the hollow body from theinterior surface of the hollow body though enforced convection. Inaddition, this effectively counteracts a possible formation of zones inwhich the flow of cooling medium is removed from the surface of theinterior surface of the hollow body (dead water zone). This isparticularly important since in these dead water zones almost no heattransfer takes place between the hollow body surface and the coolingmedium.

[0023] Intensive cooling of the hollow body results from the very highflow speed, the low temperature and the additional rotational flowwithin the hollow body. From this, a substantial production efficiencyin the blow mold method is realized by shortening the cooling times,respectively the cycle times and thus raising the product yield(piece/hour).

[0024] Especially with larger hollow bodies, through utilization of thecooling medium in circulation, a considerable cost savings is realizedthrough the use of less energy and production materials. This isamplified when utilizing even further cooled blow air of 0 C to −50 C(preferably about −30 C). This further cooled blow air must be driedprior to operation in order to prevent condensation and ice formationresulting thereform. This can for example be realized by means of anadsorption dryer. This drying process is however comparably high in costrelative to investment and use in energy. When cooling medium isutilized in a closed cycle, operating complexity for drying is loweredconsiderably in dependence on the ratio between the (lost) portions ofaeration and flushing by up to 80% depending upon the predeterminedparameters of the methods used.

[0025] The heat capacity of the cooling medium air is being raised bymeans of enrichment respectively partial or complete replacement throughother gaseous or fluid media (e.g. carbon dioxide, nitrogen, propane,noble gases or other gases that exhibit a heat capacity higher than thatof air) or other cooling means. Due to the very small loss, the closedcycling of the cooling medium according to the present invention issuitable in the foregoing case as well.

[0026] In the following table and as illustrated in FIG. 2, thepreferred blowing pressure and flow volume of blow air/cooling medium tobe utilized for various sized plastic containers is listed: PRODUCTVOLUME PRESSURE FLOW VOL. Box Hurrican   1 l 15-20 bar 0.5-1 m³/minCanister  30 l 10-15 bar   1-2 m³/min SVR fasset  50 l 10-15 bar   1-2m³/min Vanguard  136 l  9-12 bar   2-3 m³/min drum L-ring drum  220 l 9-12 bar   2-3 m³/min TC 1000 1000 l  8-10 bar   3-5 m³/min

[0027] The arrangement and the method of the invention canadvantageously be retrofitted to existing blow molding machines in asimple manner. Thereby a higher yield in terms of piece per time unitcan be realized from the blow molding machine at lower production costs.

[0028] The technical expenditure or the investment therefor arerelatively low and amortization is realized normally in a short timethrough the respective higher efficiency of the installation.

[0029] Reference Numerals

[0030]10 blow molding machine

[0031]12 blow mold

[0032]14 closed conduit system

[0033]16 blow mandrel in

[0034]18 blow mandrel out

[0035]20 connection to cooling reservoir

[0036]22 check valve

[0037]24 storage container

[0038]26 compressor

[0039]28 filter unit

[0040]30 cooler (heat exchanger)

[0041]32 check valve

[0042]34 storage container

[0043]36 blocking valve in

[0044]38 blocking valve out

[0045]40 pressure control valve

[0046]42 filter unit

[0047]44 cooler (=heat exchanger)

[0048]46 check valve

[0049]48 nozzle

[0050]50 swirl guide plate

[0051]52 dryer

[0052]54 cooling aggregate

[0053]56 switching valve

[0054]58 check valve

[0055]60 switching valve

1.) Arrangement for producing blow formed hollow bodies from plastic,with a blow molding machine (10), wherein a cooling medium can be blowninto a blow mold (12) respectively into the hollow body for blowing up(=blow-up) and cooling of the blow-molded hollow body via a conduit(14), characterized in that the blow mold (12) respectively the hollowbody is integrated into a closed conduit system (14) for guiding thecooling medium in circulation. 2.) Arrangement according to claim 1,characterized in that a compressor (26) is provided within the conduitsystem (14) for circulating the cooling medium at an elevated pressurelevel. 3.) Arrangement according to claims 1 or 2, characterized in thatat least one filter (28, 42) is provided within the conduit system (14)for cleaning the circulating cooling medium. 4.) Arrangement accordingto claims 1, 2 or 3 characterized in that at least one cooler (30, 44)is provided within the conduit line system (14) for setting apredetermined temperature of the circulating cooling medium. 5.)Arrangement according to claim 1, 2, 3 or 4, characterized in that theconduit system (14) is connected at the inlet side with a blow mandrel(16) (=blow in nozzle) for blowing in the cooling medium and at theoutlet side is connected to a further blow mandrel (18) (=outlet nozzle)for discharging the cooling medium from the blow mold (12). 6.)Arrangement according to one of the preceding claims 1 to 4,characterized in that the conduit system (14) is provided with a singleblow mandrel for special types of hollow bodies such as e.g. canisterswith a single inlet and outlet, which mandrel is provided with an inletline and a corresponding outlet line for the compressed air, wherein theinlet line and the outlet t line are disposed parallel or concentricwithin the blow mandrel. 7.) Arrangement according to one of thepreceding claims 1 to 6, characterized in that the conduit system (14)is provided with a blocking valve (36, 38) at the inlet side in front ofthe blow mandrel (16) and at the outlet side behind the blow mandrel(18) and/or in front of the inlet line and behind the outlet line forpressure release in the blow mold (12) and/or for discharging thearticle form the blow mold (12). 8.) Arrangement according to one of thepreceding claims 1 to 7 characterized in that the nozzle of the blowmandrel 916) is provided at the outlet side with a swirl body and/or aswirl guide plate (50). 9.) Arrangement according to one of thepreceding claims 1 to 8, characterized in that the conduit system (14)is provided at the outlet side behind the blow mandrel (18) with apressure control valve (4) for controlling the interior pressure in thehollow body. 10.) Arrangement according to one of the preceding claims 1to 9, characterized in that the conduit system (14) is provided with atleast one check valve (22, 32, 46) for ensuring a predeterminedcirculation direction during operation. 11.) Arrangement according toone of the preceding claims 1 through 10, characterized in that theconduit system (14) is provided with a connector (20) to a coolingmedium reservoir for replenishing any loss of cooling medium from thecooling cycle. 12.) Arrangement according to one of the preceding claims1 through 11, characterized in that a dryer (52) e.g. an adsorptiondryer, which is preferably provided with a check valve (32) isintegrated into the conduit system. 13.) Arrangement according to one ofthe preceding claims 1 to 12, characterized in that behind the dryer(52) a cooling aggregate (54) is integrated into the conduit system (14)for further cooling the cooling medium. 14.) Arrangement according toone of the preceding claims 1 to 13 characterized in that a switchingvalve having a connector is provided at a blow air pressure containerbehind the dryer (52) for supplying uncooled medium. 15.) Arrangementaccording to one of the preceding claims 1 to 13 characterized in thatthe dryer and the cooling aggregate (54) are disposed in a parallelbypass line (by-pass), which is connected to the conduit system (14) viatwo switching valves. 16.) Method for producing blow-molded hollowplastic bodies with a blow molding machine (10), wherein a coolingmedium is blown into the hollow body via a conduit (14) for forming andcooling the blow-molded hollow body, charcterized in that the coolingmedium is circulated in a closed conduit system. 17.) Method accordingto claim 16, characterized in that the cooling medium is circulated in aclosed conduit system at elevated pressure. 18.) Method according toclaim 16 or 17 characterized in that the cooling medium is circulated independence upon the size of the plastic hollow body at a pressure levelup o about 20 bar. 19.) Method according to claim 18, characterized inthat the cooling medium for plastic hollow bodies of small volume (e.g.1 l-bottle, 3 l-box) is circulated at a pressure level between 15-20bar. 20.) Method according to claim 18, characterized in that thecooling medium for the plastic hollow body of medium volume (e.g. 5l-canister, 60 l-drum) is circulated at a pressure level between 10 to15 bar. 21.) Method according to claim 18, characterized in that thecooling medium for the plastic hollow bodies of large volume (e.g. 120l-lidded drum, 220 l-tight head drum) is circulated at a pressure levelbetween 9-12 bar. 22.) Method according to claim 18, characterized inthat the cooling medium for the plastic hollow bodies with a very largevolume (e.g. 1000 l-inner containers of pallet containers) is circulatedat a pressure level between 8-10 bar. 23.) Method according to one ofthe preceding claims 16 to 22, characterized in that the cooling medium(or normally the blow air) is replaced entirely or in part by othergaseous or fluid media (such as e.g. carbon dioxide, nitrogen, propane,water, noble gases or other gases, which are characterized by a heatcapacity higher than that of air) or which is replaced or enriched byadditional cooling media. 24.) Method according to one of the precedingclaims 16 to 23, characterized in that the cooling medium is blown intothe hollow body to realize an improved cooling effect by means of aswirl body under rotational flow and swirling for improvement of heatexchange. 25.) Method according to one of the preceding claims 16 to 24characterized in that the cooling medium is dried and further cooled toa temperature between 0 C and −50 C, preferably about −30 C in order torealize an improved cooling effect prior to blowing it into the hollowbody.